In the past, the specific surface area at 400° C. of zirconia used as catalyst supports was about 100 m2/g. In addition, zirconia having a larger specific surface area was typically amorphous without having a constant structure. Consequently, in the case of using zirconia as a catalyst support, stable performance at high temperatures is unable to be obtained as a result of the specific surface area thereof decreasing at temperatures equal to or higher than 400° C. Thus, it is necessary to improve the heat resistance (thermal stability) at high temperatures in order to use zirconia as a catalyst support.
In contrast, zirconia-ceria compositions composed of zirconium oxide and cerium oxide have higher heat resistance than zirconia in that they are able to ensure a comparatively large specific surface area even at a high temperature of 1000° C.
Recently, heat resistance of pore volume has also come to be required in addition to heat resistance of specific surface area of catalyst supports. In the case of loading an active species in the form of a precious metal (active previous metal) onto a catalyst support, the precious metal is loaded with good dispersibility into pores have a diameter of 10 to 100 nm. Thus, a catalyst support having a large pore volume consisting of pores having a diameter of 10 to 100 nm even at high temperatures is desirable. Namely, heat resistance is required such that pores have a diameter of 10 to 100 nm even at high temperatures.
Published Japanese Translation No. 2001-524918 of a PCT International Publication describes a cerium oxide, zirconium oxide, (Ce,Zr)O2 mixed oxide and (Ce,Zr)O2 solid solution having a total pore volume in excess of about 0.8 ml/g following calcining in air for 2 hours at about 500° C.
Japanese Patent No. 3016865 describes a mixed cerium or zirconium oxide having a total pore volume of at least 0.6 cm3/g and composed of pores in which at least 50% of the total pore volume has a diameter of 10 to 100 nm. In addition, a mixed oxide having a pore volume of about 0.8 cm3/g following calcining for 6 hours at 800° C. is described in the examples thereof.
However, when considering that the temperature at which automotive catalysts are actually used is 1000° C. or higher, the mixed oxides described in the two publications mentioned above are unlikely to have adequate heat resistance at high temperatures.
On the other hand, Japanese Patent Application Laid-open No. 2006-36576 describes a zirconia porous body and a production method thereof. More specifically, a zirconia porous body is described having a peak pore diameter of 20 to 110 nm for a pore size distribution as determined by the BJH method, and a total pore volume of not less than 0.4 cc/g in which the total volume of pores having a diameter of 10 to 100 nm accounts for 50% or more of total pore volume.
In addition, the production method consists of a method for producing a zirconia porous body comprising the steps of: (1) a first step of mixing a basic zirconium sulfate-containing reaction liquid A, prepared by mixing a sulfating agent (reagent for forming sulfate) at 80° C. to less than 95° C. and a zirconium salt solution at 80° C. to less than 95° C., with a basic zirconium sulfate-containing reaction liquid B, prepared by mixing a sulfating agent at 65° C. to less than 80° C. and a zirconium salt solution at 65° C. to less than 80° C., (2) a second step of aging the reaction liquid obtained in the first step at 95° C. or higher, (3) a third step of forming zirconium hydroxide by adding an alkali to the mixed liquid obtained in the second step to neutralize the basic zirconium sulfate, and (4) a fourth step of heat-treating the zirconium hydroxide to obtain a zirconia porous body.
The examples of Japanese Patent Application Laid-open No. 2006-36576 describe that the maximum value of total pore volume following heat treatment for 3 hours at 1000° C. is 0.7 cc/g. In this manner, although the heat resistance of total pore volume has been improved, further improvement is still required.